The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical diagnostic magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated that the present invention also finds application in magnetic resonance spectroscopy, magnetic resonance imaging for other applications, and generally those applications where homogeneous magnetic fields are desirable.
Generally, nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) techniques employ a spatially uniform and temporally constant main magnetic field, B.sub.0, generated through an examination region. Superimposed on the B.sub.0 magnetic field is a B.sub.1 radio frequency (RF) magnetic field at the NMR resonant frequency. For MRI applications, there is also a set of gradient magnetic fields used to spatially encode resonant spins. Some MRI techniques are highly sensitive to magnetic field homogeneity. However, the geometric shape and/or magnetic susceptibility of a subject being scanned, built-in main magnet tolerances, environmental and/or site effects, and the like contribute to the main magnetic field's inhomogeneity and/or non-uniformity. In turn, this leads to imaging problems.
Methods for controlling the homogeneity of the main magnetic field include both passive and active shimming techniques. The passive technique is typified by arranging shim steel to minimize static magnetic field inhomogeneities based upon NMR field plot measurements. The NMR field plot measurements are performed without a subject in the examination region. Generally, the shim steel technique is not adjustable on a scan-by-scan basis. It is mainly used to shim out the effects of built-in magnet tolerances and environmental or site effects. This technique is not suited to handle inhomogeneities within the main magnetic caused by subject geometry and/or susceptibility.
Active shimming generally employs multiple orthogonal shim coils and/or gradient coil offsets. An electrical current is applied to the shim coils and/or gradient coil offsets in order to cancel inhomogeneities in the main magnetic field. In some cases, initial optimal shim currents are applied to the shim coils to initially establish uniform magnetic fields using the same type of NMR field plot measurements described above with reference to the passive technique. Commonly first order and occasionally second or third order corrections are implemented to compensate for non-uniformities in the main magnetic field. Furthermore, the shimming can be adjusted from scan-to-scan and/or subject-to-subject to adjust for inhomogeneities caused by the susceptibility and/or geometric shape of a subject being imaged. In general, there are a number of methods and/or techniques of active shimming aimed at homogenizing the main magnetic field in an MRI system. These techniques and methods can broadly be grouped as follows: signal maximization methods, phase-fitting algorithms, field mapping techniques, peak location/time methods, chemical shift imaging methods, measurement of distortion of known objects, and various qualitative methods to view field homogeneity which do not compute shim term corrections.
In one particular prior art reference, U.S. Pat. No. 5,359,289 to van der Meulen, a peak location/time method is proposed. The method measures the time location of the peak of the magnetic resonance signal (in one example, a gradient echo and in another, a spin echo) and compares the measured time location of the peak against a theoretically predicted time location to produce a linear shim term correction. However, various subtle timing delays may exist in the gradient and/or other system components which are independent of the sampling system. It is assumed that such delays or errors are negligibly small. This may not necessarily be the case, and when such errors are not negligibly small, the shim technique based strictly on timing measurements results in inaccurate shimming.
In another particular reference, U.S. Pat. No. 5,391,990 to Schmitt, et al., shimming is accomplished via the use of an echo planar sequence having a sinusoidal readout with the phase encode lobes disabled. The algorithm observes the even versus odd echo timing throughout a number of echoes of data acquisition and determines a best fit shim value which aligns the echoes. However, a high band width is required to sample the data, and the accuracy of the peak positioning algorithm may be compromised by the presence of noise. Additionally, with such data, the echo positions do not necessarily progress in a linear fashion or higher order drift, but rather may tend to wobble temporally. The gradient amplifier duty cycle, and the like may induce this temporal wobble in the signal. Furthermore, the reference fails to account for miscellaneous timing delays between various gradient axes which can also cause the peaks to not co-register correctly in oblique scans.
The present invention contemplates a new and improved shim algorithm for use in the magnetic resonance applications which overcomes the above reference problems and others.